The Importance of Engineering in Ecosystem Restoration

Here’s a streambank restoration project on a Cuyahoga River tributary that, from the photo, looks to have been missing some important engineering.  The designer placed a woven drainage filter fabric on a steep, excavated bank, and installed rounded cobbles against the fabric to restore the eroded streambank. The streambank restoration failed during a subsequent flooding event, destroying the repair and exposing bare banks that will contribute detrimental sediment to the aquatic ecosystem.  This failure, like many, can be attributed to at least three factors: the rock; the filter drainage fabric; and the toe protection.  

Rock used to protect a streambank must be engineered because the stability of the rock must conform to the laws of physics.  Those laws, when applied to streambanks, account for: the median size of the stone (D ₅₀), specific gravity of the stone, specific gravity of water, a metric to represent the uniformity of the gradation of the stone, the relationship between the thickness of the layer and the largest (D₁₀₀ ) particle size, channel bend radius, channel bottom width, water depth, bank slope, stream velocity, and a stone stability factor.  The stability factor varies depending on whether the stone is angular, typical of quarried rock, or rounded, typical of glacial material found in a gravel pit.  Studies have shown that rounded stone is 20% less stable than angular stone of the same size and specific gravity.  The angularity naturally interlocks, tying the entire rock layer together.      

Drainage fabric is available in two types: woven and non-woven.  The woven fabric is thin, smooth, and slippery.  The non-woven is thick, soft, and spongy.  In this application, the woven fabric could have contributed to the failure due to its slippery properties.  Better yet, the fabric could have been eliminated entirely and replaced with either an engineered bedding layer of smaller size stone, or by using a thicker layer of armoring stone that is self-filtering. 

The photo does not indicate whether some type of stone toe protection was provided.  All streambank restoration projects should have either a launchable stone toe or a stone toe that’s excavated into the channel bed at the toe of the slope to prevent failure of the bank when scour occurs in the channel, and it will scour.   Although scour at the toe of a slope is most likely to occur at the outside of a bend, in this photo the stream is flowing into the picture, and the failed streambank protection is located in a straight section of the channel just upstream of a bend.  Even straight sections of channel will experience toe scour and should have a stone toe.

On riverbank restoration projects, therefore, it is essential to do the engineering studies to properly design the restoration to provide long term stability to the streambank so that the aquatic, cultural and other resources can be preserved.